Fuel cell is a power supplying apparatus that utilizes chemical fuels to conduct an oxidation/reduction reaction instead of a conventional combustion reaction to transfer chemical energy into electronic energy, producing clean exhaust in a safe and efficient manner. So, the fuel cell has become one new alternative for the fuel supply. On the other hand, a timely fuel replenishing is all it requires when the fuel cell is used as a power supply device. It is different from conventional rechargeable battery that requires continuous charging to maintain power supply. Thus, the fuel cell can be applicable to both stationary home appliances such as illuminating apparatus, televisions or refrigerators and portable electrical appliances such as vehicles, mobile phones, and laptops.
For a direct methanol fuel cell (DMFC), an anode of membrane electrolyte assembly (MEA) is immersed in a methanol solution to allow an oxidation-reduction reaction of the methanol on the anode, so as to transfer chemical energy into electronic energy. However, operation performance of the direct methanol fuel cell is highly associated with methanol concentration in its fuel tank. When the methanol concentration in the fuel tank is too low, the cell potential is reduced resulting in poor operation performance for the whole fuel cell. On the other hand, a highly concentrated methanol permeates a proton exchange membrane of the MEA to the cathode, causing a phenomenon called “fuel crossover” in which the methanol is oxidized at the cathode causing a practical loss of methanol fuel and poisoning of the cathode. As a result, a loss of operation performance is encountered. Thus, methods for maintaining the fuel concentration in the fuel tank of the DMFC within a certain range are critical factors to improve operation performance of fuel cells.
In addition, methanol used in direct methanol fuel cells is a volatile organic solvent that is highly toxic and inflammable which poses certain levels of hazard in terms of its storage, while it is not convenient to carry and add the liquid methanol. Thus, researches directed to DMFC have focused on ways to improve convenience in carrying and adding methanol as well as to reduce hazardous level associated with methanol fuel storage.
Japan Laid Open Patent No. 2001-185185 and 2001-291523 both disclose a fuel cell requiring additional devices and tubes to control the exhausted carbon dioxide, water, and the concentration of the fuel, so as to maintain certain performance for the fuel cell. However, additional devices may undesirably increase the overall cost and size for the fuel cell, making the fuel cell of this type not suitable for small sized electronic devices and portable electronic devices. Furthermore, U.S. Pat. No. 6,440,594 discloses a direct methanol fuel cell in which the fuel is provided in the form of an aerosol of liquid fuel droplets suspended in a gas to solve the foregoing drawbacks. However neither characteristics of the materials, nor control for rate of the fuel release is specified in this patent.